This invention relates to a helical scan type video data playback apparatus, and more particularly to a video data playback apparatus capable of satisfactorily playing back or reproducing video data recorded on a magnetic tape by the low frequency band color recording system.
In a conventional helical scan type magnetic playback apparatus including a plurality of magnetic heads arranged on a rotary drum, an approach is employed to alternately switch every field playback video signals from the plurality of magnetic heads, e.g., two magnetic heads in dependency upon a rotational position of the rotary drum to play back a switched video signal. Such a switching is called a "head switching". Playback based on this switching is conducted to play back video signals from two successive tracks in a manner that they overlap with each other by winding a video tape onto the rotary drum over an angular range of 180-odd degrees to switch them in synchronism with the detection of a rotational position of the rotary drum. This switching is conducted at the time of playing back the lower part of a frame so that noises on a picture due to the switching become inconspicuous.
However, because a tension of a video tape at the time of recording and that at the time of playback are different from each other, or for other reasons, the following problems exist: One is skew on a playback frame, i.e., shift of a horizontal position of an image of an object played back or reproduced on a frame from the point where head switching is conducted (switching point); the other is shift of the switching point in dependency upon a tracking adjustment when that adjustments is conducted. Further problem is as follows. Since televisions manufactured, etc. are subjected to various adjustments so that a viewer can observe the lower part of a picture as a result of an improvement in recent televisions and video monitors, noises on a picture due to the above-mentioned switching become conspicuous. For example, the switching point is ordinarily set at 6.5H (horizontal scanning interval) before the vertical synchronizing signal. However, there are instances where the switching point is shifted therefrom about .+-.3H. Since the switching position appears within a frame at 8 to 4.5H immediately before vertical synchronization, the place where head switching is conducted would enter that frame.
Additional problem is as follows. At the time of playing back a video tape recorded by other VTRs, the switching point might shift depending upon at which position in a width direction of the tape a video signal is recorded, resulting in that the switching point enters further inside of the frame.